Initial toolbox.py commit.

This commit sets up some basic structures in toolbox.py, although most of them are not yet used.

A function `find_project_file(path)` attempts to find a Toolbox project file at `path` (whether `path` is a file or a directory containing a .prj file).

A second function `open_toolbox_file(fh)` reads a Toolbox-formatted file and returns pairs of (marker, value). This function does not have much error handling for malformed input, and doesn't do anything intelligent for the XML-like Toolbox structures (e.g. \+corpus ... \-corpus).